This invention relates to a combinatorial weighing system, particularly a combinatorial weighing method and apparatus for a case where weighing is executed to obtain a target weight value greater than the maximum load of the apparatus, wherein the target value can be weighed out through only a small number of weighing operations.
A combinatorial weighing apparatus has a plurality of weighing machines each consisting of a weighing hopper and a weight sensor associated with the weighing hopper. According to a known combinatorial weighing method using the aforesaid apparatus, combinatorial weighing is carried out by weighing articles which have been introduced into the weighing hoppers of the weighing machines, selecting the combination of weighing machines (referred to as the "optimum" combination) that gives a total weight value equal to a target weight value or closest to the target weight value within preset allowable limits, discharging only those articles contained by the weighing hoppers of the selected weighing machines, and subsequently replenishing the emptied weighing hoppers with new articles to prepare for the next weighing cycle. The foregoing sequence of steps is repeated to carry out a continuous, highly accurate weighing operation automatically.
In an automatic weighing apparatus that operates based on the above-described combinatorial weighing method, the target weight value is determined within a substantially constant range and the optimum hopper capacity is selected as an apparatus design specification within this range. In addition, the quantity of articles supplied to each weighing hopper and the supply timing are adjusted in advance in such a manner that the combination obtained will give the optimum precision, as will be described later. However, in one and the same production line, certain conditions in the production process give rise to situations where weighing must be carried out using a target weight value which is greater (several, times greater in some instances) than the above-mentioned usual target weight value. Although such situations may be temporary, they occur with some frequency.
In order to perform weighing with a combinatorial weighing apparatus where the weight of the articles to be weighed out exceeds the weighing capacity of the apparatus, it is common practice to either: (A) divide a target weight value into a number of weight values X1, X2, X3 . . . each of which is less than the maximum weighing capacity of the apparatus, and then simply repeat the combinatorial weighing operation a plurality of times; or (B) divide the target weight into a number of weight values, each of which is less than the maximum weighing capacity of the apparatus and then, in conducting weighing from the second weighing operation onward, correct the target weights X2, X3, . . , which will prevail in the respective weighing operations, by any error in the results of the preceding weighing operation.
Weighing method (B) outlined above will now be described in greater detail with reference to the flowchart of FIG. 1. We will assume that the target weight value is 3X grams, and that X1=X, X2=X, X3=X. In order to weigh out 3X grams of the articles, method (B) proceeds in the following fashion:
(1) First, all of the weighing machines are supplied with articles to be weighed.
(2) The weights of the artcles fed into the weighing hoppers of the weighing machnes are measured (first weight measurement).
(3) Combinations are compuied with X1 (=X) grams serving as the target, and the difference between X and Y1 (which is the total weight value of the articles contained by those weighing machines that give the optimum combination), is stored in memory as an error E1 (=Y1-X).
(4) The articles are discharged from the weighing machines that give the optimum combination (first discharge operation).
(5) The emptied weighing hoppers of the weighing machines, that is, those that have discharged their articles, are supplied with articles afresh.
(6) The weights of the articles fed into each of the weighing hoppers of the weighing machines are measured (second weight measurement).
(7) Combinations are computed with X2-E1 (=X-E1) grams serving as the target, and the difference between the target value (X-E1) and Y2 (which is the total weight value of the articles contained by those weighing machines that give the optimum combination), is stored in memory as an error E2(+Y1+Y2-2X). It should be noted that: ##EQU1##
(8) The articles are discharged from the weighing machines that give the optimum combination (second discharge operation).
(9) The weighing hoppers of the weighing machines that have discharged their articles are supplied with articles afresh.
(10) The weights of the articles fed into each of the weighing hoppers of the weighing machines are measured (third weight measurement).
(11) Combinations are computed with X3-E2 (=X-E2) grams serving as the target, and the articles are discharged from the weighing machines that give the optimum combination (third discharge operation). The end result is 3X grams of the articles.
A disadvantage encountered with the above-described target weight dividing method when weighing out articles to a weight greater than the maximum weighing capacity is that combinatorial computations must be performed a considerable number of times to obtain a target weight above the maximum weighing capacity, so that the method is not suitable for weighing at high speed. Another problem with the foregoing combinatorial weighing method is that there are instances where some weighing machines remain unselected for a prolonged period of time so that the weighing hoppers thereof retain their articles for too long. The reason for prolonged retention of articles in a weighing hopper is that the article batch has a peculiar weight which does not lend itself to selection. If article batches having peculiar weights grow in number because they are unfit for selection, a situation will eventually arise in which no desirable combinations can be obtained. Furthermore, articles such as frozen foods will thaw or spoil if retained in the weighing hoppers for an extended period of time. It is obvious, therefore, that prolonged retention of articles in unselected weighing hoppers is undesirable and should be avoided.